1. Field of the Invention
The present invention relates to a surface emitting laser device, a surface emitting laser array, an optical scanning device, an image forming apparatus, and a manufacturing method of the surface emitting laser device. More particularly, the present invention relates to a surface emitting laser device and a surface emitting laser array capable of emitting light in the direction orthogonal to their substrate surfaces, an optical scanning device including the surface emitting laser device or the surface emitting laser array, an image forming apparatus having the optical scanning device, and a manufacturing method of manufacturing the surface emitting laser device capable of emitting light in the direction orthogonal to its substrate surface.
2. Description of the Related Art
A Vertical Cavity Surface Emitting Laser (hereinafter may be referred to as “VCSEL”) is capable of emitting light in the direction orthogonal to its substrate. When compared with edge emitting semiconductor lasers capable of emitting light in the direction parallel to its substrate, the VCSEL may have some advantages such as lower cost, lower energy consumption, smaller size, preferable for two-dimensionally integrated devices, and higher performance. Recently, because of those advantages, the VCSEL has attracted increased attention.
The surface emitting laser has a current confined structure to enhance current influx efficiency. To form the current confined structure, a selective oxidation process is usually performed on an AlAs (Al: Aluminum, As: Arsenic) layer. In the following, the current confined structure may also be referred to as an “oxide-confined structure” for convenience.
The oxide-confined structure may be formed by forming a mesa structure having predetermined sizes and having a side surface where a selectively-oxidized layer made of p-AlAs is exposed. Then, the formed mesa structure is processed under a water-vapor atmosphere so that aluminum (Al) in the selectively-oxidized layer is selectively oxidized from the side surface of the mesa structure. By doing this, an unoxidized region remains in the center portion of the mesa structure. The unoxidized region (hereinafter referred to as a “confined region” for explanatory purposes) becomes a passing region (or a “current passage region”) through which a driving current of the surface emitting laser passes. As described above, the current may be easily confined. The refractive index of the aluminum-oxidized layer (AlxOy) (hereinafter simplified as an “oxidized layer”) in the oxide-confined structure is approximately 1.6, which is lower than that of semiconductor layers. Because of this feature, a refractive index difference is generated in the lateral direction in a resonator structure of the surface emitting laser, and the light is confined in the center of the mesa structure, thereby improving the emission efficiency of the surface emitting laser. As a result, it becomes possible to obtain excellent characteristics such as lower threshold current and higher efficiency.
The surface emitting laser may be generally applied to a light source of an optical writing system in a printer (oscillation wavelength: 780 nm band), a light source of an optical writing system in an optical disk device (oscillation wavelength: 780 nm band and 850 nm band), and a light source of an optical transmission system such as a LAN (Local Area Network) using optical fibers (oscillation wavelength: 1.3 μm band and 1.5 μm band). Further, the surface emitting laser is also expected to be used as a light source for optical transmission between boards, within a board, and between chips and within a chip in a Large Scale Integrated circuit (LSI).
In those application fields, it is generally required that a cross-sectional shape of the light emitted from the surface emitting laser (hereinafter referred to as “emitting light”) be circular. To achieve the circular cross-sectional shape, it is required to control higher-order transverse-mode oscillation.
For example, Japanese Patent Application Publication No. 2001-156395 (Patent Document 1) discloses a surface emitting laser device where a semiconductor material layer structure in which a light emitting layer is disposed between an upper reflection mirror structure and a lower reflection mirror structure is formed on a substrate, an upper electrode having an annular shape when viewed from the top is formed on the upper side of the upper reflection mirror structure, an opening is formed on the inner side of the upper electrode, and a part of the surface of the opening is coated with a layer transparent to the oscillation wavelength of the oscillated laser light.
Further, Japanese Patent Application Publication No. 2006-210429 (Patent Document 2) discloses a surface emitting laser device including an active layer having a light emitting center region, a pair of multi-film reflection mirrors sandwiching the active layer, one of the multi-film reflection mirrors having a light emitting region, an electrode having an opening part corresponding to the light emitting region, and an insulating film formed to be corresponding to the light emitting region in a manner such that the reflection rate in a peripheral part surrounding a center part corresponding to the light emitting center region is lower than the reflection rate in the center part. Further, in the part corresponding to the center part of the light emitting region, the insulating film is made of a structure where a first insulating film and a second insulating film are laminated. Further, in the part corresponding to the peripheral part of the light emitting region, there is a third insulating film having a refractive index lower than that of the first insulating film.